The present invention relates in general to dehumidifying air conditioning systems, and relates in particular to a desiccant assisted air conditioning system to provide continuous processes of desiccant-assisted dehumidification and desiccant-regeneration using a heat source.
FIG. 10 shows a conventional dehumidifying air conditioning system having a process air path for dehumidifying air by passing the air through a desiccant, and a regeneration air path for desorbing moisture from the desiccant by passing heated air through the desiccant, arranged in such a way to flow the process air and regeneration air alternatingly through the desiccant. The system comprises: a process air path A; a regeneration air path B; a desiccant wheel 103; two sensible heat exchangers 104, 121; a heater 220; and a humidifier 105. Process air is dehumidified in the desiccant wheel 103, and, in this process, is heated by the heat of adsorption of moisture in the desiccant member, and is cooled next by passing through a first heat exchanger 104 by exchanging heat with the regeneration air. Process air is further cooled in the humidifier 105 before being supplied to the conditioning space (room) supply air SA. In the meantime, outside air (OA) serving as regeneration air is admitted into the first sensible heat exchanger 104 which raises the temperature of regeneration air by transferring heat from the dehumidified process air, and the heated regeneration air is further heated by a heat source 200 in the heating device 220 to lower its relative humidity, and is passed through the desiccant wheel 103 to desorb the moisture from the desiccant member. In the conventional system, sensible heat portion in the post-regeneration regeneration air is recovered by heat exchange with unheated regeneration air in the second sensible heat exchanger 121, before exhausting the regeneration air to outside (EX). This type of system is known as a desiccant-assisted air conditioning system, and is an important practical technique to provide control over conditioning space humidity.
Desiccant materials which can be used in such desiccant-assisted air conditioning systems are known to include silica-gel and zeolite (known as molecular sieve), which are classified as a modified zeolite in Breuner type 1. It is said that those materials having an isothermal separation factor in the range of 0.07xcx9c0.5 are most suitable as a desiccant member which is used in those systems designed to carry out desiccant regeneration by using some combustible gas as heat source. U.S. Pat. No. 3,844,737 mentions zeolite as a desiccant material in air conditioning systems using combustible gases for heating regeneration air, but, no prior publications give any suggestions regarding the suitable adsorption characteristics of zeolite. Although lithium chloride has also been used as a moisture adsorbing material, its use has gradually been discontinued because of deliquescence tendency when exposed to high humidity to fall out from a rotating frame of the desiccant wheel.
In air conditioning technologies based on combustible gas heating of regeneration air, as mentioned above, regeneration temperature is reported as 101xc2x0 C. (215xc2x0 F. ) or 143xc2x0 C. (290xc2x0 F.). It is said that zeolite is suitable for regeneration at such temperatures, and in particular, zeolite having an isothermal separation factor R between 0.07xcx9c0.5 as exemplified by R=0.1 in FIG. 11 is most suitable. However, if other types of heating sources are considered for desiccant regeneration, lower regeneration temperatures (65xcx9c75xc2x0 C.) offer more available choices, such as waste heat and solar heating. But, in such a case, zeolite materials in Breuner type 1 class and having a separation factor in the range of 0.07xcx9c0.5 are not always an optimum material for desiccant. The reason will be explained with reference to FIG. 11.
FIG. 11 is an adsorption isotherm of conventional zeolite. When outdoor air is used as regeneration air in a desiccant-assisted air conditioning system, humidity ratio in summer is estimated to be about 20xcx9c21 g/kg (g moisture/kg air) for design purposes. When such an air is heated to a desiccant desorption temperature of 110xc2x0 C. mentioned above, its relative humidity drops to about 3.0%. On the other hand, relative humidity of process air to be dehumidified can be estimated to be about 50% based on general room conditions where dry-bulb temperature is 27xc2x0 C. and wet-bulb temperature is 19xc2x0 C. as specified in JIS(Japanese Industrial Standard)-C9612, for example. The desiccant member thus alternatingly contacts process air and regeneration air, respectively, at 50% and 3% relative humidity. Equilibrium moisture content in zeolite in contact with regeneration air at 3% relative humidity is found to be X=0.236 from FIG. 11, using a functional relation X=P/(R+Pxe2x88x92Rxc3x97P) for a separation factor R=0.1 and P=0.030.
On the other hand, equilibrium moisture content in zeolite in contact with process air exhausted from a room can be found, similarly, to be X=0.910 for separation factor R=0.1 and P=0.5. Therefore, in the case of heating the regeneration air to 101xc2x0 C. for desorbing zeolite, the amount of moisture which can be adsorbed by the desiccant member is 0.169 kg/kg, which is obtained by multiplying the difference in the relative adsorbed amount (0.910xe2x88x920.236=0.674) with the maximum uptake 0.25 kg/kg (kg water per kg zeolite). If a material such as silica-gel is used, whose characteristic adsorption isotherm is linear (isothermal separation factor R=1), the difference in desorption and adsorption is the same as the difference in the relative humidity values, 0.500xe2x88x920.030=0.470, and a corresponding value drops to 0.140 kg/kg, which is obtained by multiplying the maximum uptake (usually 0.3 kg/kg for silica-gel) with 0.470. Therefore, zeolite is more effective in this case. This example shows that, when the desorption temperature is as high as 101xc2x0 C. as in the conventional air conditioning apparatus, the use of zeolite is clearly more advantageous. However, when similar calculations are performed for the range of desorption temperatures of 50xcx9c70xc2x0 C. as desired in the present invention, superiority of zeolite is not certain and the differential adsorption capacity (difference in desorbed/adsorbed amount) is significantly decreased. This will be explained in more detail below with reference to FIG. 12.
FIG. 12 shows the configuration of a desiccant-assisted air conditioning system disclosed the inventor, comprised by a process air path for dehumidifying and a regeneration air path for flowing air which is first heated in a heating source before desorbing moisture from the moisture-laden desiccant member 103, arranged in such a way that regeneration air and process air alternatingly flow through the desiccant member 103. Dehumidified process air is cooled in a low-temperature heat source 240 of a heat pump, and pre-desiccant regeneration air is heated in a high-temperature heat source 220 of the heat pump. FIG. 13 shows a psychrometric chart to show the operation of the system shown in FIG. 12.
Accordingly, by cooling the dehumidified process air in the low-temperature source 240 of the heat pump, the temperature of supply air SA (state N) can be lowered below that of the room (state K) as shown in FIG. 13. Therefore the humidifier 105 used in the conventional system shown in FIG. 10 becomes unnecessary so that dehumidified cooled process air and supply air SA have the same humidity ratio, thus providing a higher cooling effect than the conventional system. Those skilled in the art know that, for summer air conditioning, supply air is generally at less than 8 g/kg (moisture per kg of air), therefore, by setting the humidity ratio of the supply air, i.e., dehumidified process air at 7 g/kg, the process air changes its state from the room state along an isenthalpic line until it reaches 7 g/kg where a relative humidity is 20%, as shown in FIG. 13 (when the adsorption heat is high as in zeolite, relative humidity of 20% is reached at a slightly higher humidity ratio value of 8 g/kg).
It is known by those skilled in the art that the relative humidity of dehumidified process air is equal to the relative humidity of regeneration air before regeneration (for example, refer to reference material p23xcx9c25 of TC 3.5/short course seminar, US ASHRAE Society Annual Meeting, 1997). Therefore, outdoor air can be heated to a temperature to lower its relative humidity so as to be used as regeneration air to regenerate the desiccant member.
In other words, humidity ratio in summer is generally about 15 g/kg, therefore, such an air, when heated to 50xc2x0 C. having a 20% relative humidity, can be used as regeneration air. Humidity ratio can reach a value of 20 g/kg on rare occasions, but even such an air can be heated to 55xc2x0 C. and used for dehumidifying the process air to less than 8 g/kg moisture. Therefore, it is desirable for such an air conditioning system to have a desiccant material which provides a high moisture removal capacity at regeneration temperature of 50xcx9c70xc2x0 C., but the conventional zeolite shows a low capacity for moisture content difference between its absorption and desorption state. Thus, low capacity must be compensated by increasing the mass of the desiccant. This will be explained in more detail below.
When the regeneration air at humidity ratio of 15 g/kg is heated to 50xc2x0 C., its relative humidity is about 20% (18.9% accurately). Therefore, equilibrium moisture content of zeolite of separation factor R=0.1 in contact with regeneration air is X=0.71 for P=0.2 when relative humidity is 20% as shown in the graph in FIG. 11. On the other hand, equilibrium moisture content of zeolite in contact with spent process air exhausted from the room is X=0.91 at P=0.5 as before. Therefore, by flowing regeneration air heated to 50xc2x0 C., the desiccant can adsorb moisture of 0.05 kg/kg, obtained by multiplying the differential adsorption capacity 0.20 (=0.91xe2x88x920.71) with the maximum uptake of 0.25 kg/kg for zeolite. Comparing this value 0.05 with the previous value 0.169 kg/kg, gives a ratio as 1/3.4, which means that the size of the zeolite desiccant needs to be 3.4 times larger.
FIG. 14 is a graph, calculated from the adsorption isotherm of FIG. 11, showing the relationship between adsorption capacity of zeolite and temperature of air in contact therewith for various parametric values of the humidity ratio of the air. Point A is the adsorption-start point where the moisture content of zeolite is in equilibrium with process air, and points D50 and D70 are the desorption- or regeneration-start points where the moisture content of zeolite is in equilibrium with regeneration air at 50 and 70xc2x0 C., respectively. This graph also shows that the differential adsorption capacity is 0.05 and 0.11 kg/kg, respectively, for 50xc2x0 C.-regeneration and 70xc2x0 C.-regeneration. These values confirm that the desiccant size must be increased by 1.5xcx9c3.4 times the size of a desiccant regenerated at higher temperatures.
On the other hand, if a material such as silica-gel is used, whose adsorption isotherm is linear (separation factor R=1), the differential adsorption capacity is 0.3 (=0.5xe2x88x920.2) for 50xc2x0 C.-regeneration (relative humidity 20%), similarly to the differential relative humidity, so that, adsorbed amount is 0.09 kg/kg, obtained by multiplying 0.3 with the maximum uptake of 0.3 kg/kg for silica-gel. For 70xc2x0 C.-regeneration (relative humidity 7.5%), the adsorbed amount is 0.127 kg/kg, obtained by multiplying the differential adsorption capacity 0.425 (=0.5xe2x88x920.075) with the maximum uptake of 0.3 kg/kg for silica-gel. These values (0.09, 0.127) are higher than those for zeolite type 1 (0.05, 0.11), but even in these cases, it is clear that the desiccant size must be increased compared with the high-temperature regeneration process which produces an adsorption amount of 0.14 kg/kg.
It can be seen, therefore, that the conventional desiccant technology is not adaptable to low-temperature regeneration (50xcx9c70xc2x0 C.), and the necessity for a larger desiccant leads to a large air conditioning system and high operating cost.
It is an object of the present invention to provide a compact and energy efficient air conditioning system that is operated with a desiccant material having a high differential adsorption capacity even at lower regeneration temperatures than those in the conventional system.
A desiccant assisted air conditioning system comprises: a process air path for flowing process air to adsorb moisture from the process air by a desiccant member; and a regeneration air path for flowing regeneration air heated by a heat source to desorb moisture from the desiccant member, the desiccant member being arranged so that the process air or the regeneration air flows alternatingly through the desiccant member; wherein the desiccant member comprises an organic polymer material, the organic polymer material comprising an amphoteric ion exchange polymer having an anion exchange group, a cation exchange group and bridging ligands, thereby exhibiting a high differential adsorption capacity.
An air conditioning system using such a desiccant material permitting regeneration at relatively low temperatures (50xcx9c70xc2x0 C.) enables to provide a compact and energy efficient air conditioning system.
The organic polymer material is obtained by reacting an acrylonitrile homopolymer or copolymer with a hydrazine or hydrazine homologue to provide an anion exchange group followed by hydrolyzing residual nitrile group to provide a cation exchange group.
The organic polymer material thus produced can be used as a desiccant material in the air conditioning system that is energy conserving and compact because the desiccant medium can be regenerated at 50xcx9c70xc2x0 C.
The organic polymer material may include the anion exchange group at a concentration of 0.01xcx9c5.0 meq/g and the cation exchange group at a concentration of 2xcx9c11 meq/g.
The desiccant medium having the properties so defined exhibits a high deferential moisture adsorption capacity, to enable a compact high energy efficiency system.
The desiccant material may be regenerated at a temperature of not more than 70xc2x0 C.
Because the desiccant medium allows regeneration at low temperatures, heat pump can be operated at relatively low temperatures, thereby enabling to provide a compact and high efficiency air conditioning system.
Dehumidified process air is cooled by a low-temperature heat source of a heat pump, and pre-desiccant regeneration air is heated with a high-temperature source of the heat pump.
Accordingly, regeneration air is heated by recycling the heat recovered from dehumidified process air, thereby enabling to maximize utilization of output heat from the heat pump, and also enabling to decrease the temperature lift required for the heat pump, thereby providing a compact and energy efficient air conditioning system.